IMTU humanity and numerous other races (mostly carbon based oxygen breathers) have been around for millenia so there's an abundance of worlds out there capable of supporting indigenous life, usually one or two per quadrant. The remaining worlds that are not wxotic atmosphere types or in vacuum I postulate that they have most likely been terraformed or at least partly terraformed by the Villani that being a conservative culture and a bunch of cultural imperialists wanted to shape the galaxy in their own image and didnt mind if the process took centuries or even millenia, as after all investment ultimately meant profits for the merchant and banking types even if those profits were not likely to come rolling in for a few centuries. The villani bureaus could deal with such long term investment as they expected to last forever. The coming of the terrans and the resultant decay of the 2nd Imperium and the Long night saw many of the terraforming projects fall into a state of little or no maintenance creating a degredation in the conditions of the many and various colony worlds.
This suits the excellent MT World Builders Handbook totally as worlds detailed with it in the habitable zone are usually freezing cold or boiling hot hell holes. Thanks to the villani most worlds have life though its not necessarily indigenous and have established or runaway ecosystems.
The way I see it when visiting a solar system for the first time, its always the outerbody worlds that are going to be the easiest to terraform as they are going to be naturally cooler than inner system worlds. Basic science tells you that it will always be easier to create a greenhouse effect or thicken an atmosphere in order to add heat to a world as opposed to trying to cool an atmosphere or thin it somewhat in the inner system.
We have the technology today to terraform mars and a variety of other outer system bodies (we don't have the money or the will though - which is sad
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As previously mentioned if earth didnt exist then the terraformers would simply look for the next best thing.
Also its wildly assumed that habitable worlds with a standard, dense or thin atmosphere have oxygen in sufficient quantitie to support terran life-forms (this is unrealistic - hence a terraforming story has to be built into the traveller history). What I would like to propose is that from now on we just use the UWP atmosphere code as an indication of pressure not atmospheric content. As always the referee should decide whats in the atmosphere and if anything can live in it.
Personally I would like to see many more cities under domes or sealed within airtight caverns or fissures etc or nestling next to pure oxygen geasers which would give traveller much more of a frontier feel to it.
The most exciting adventure's I've personally enjoyed have been whenever the group has strayed away from an earth normal 'garden world' and had to cope in vacuum or frozen carbon dioxide snow in a pure nitrogen atmosphere etc. It's what space travel and by definition Traveller is all about. It also neetly explains why most people never leave their worlds and habitats, put simply space and the worlds it contains are damn dangerous, though tempting enough for those in search of a profit or revelation or discovery etc.
